1. Field of the Invention
One disclosed aspect of the embodiments relates to a two-dimensional absolute encoder and a scale.
2. Description of the Related Art
Incremental and absolute encoders have been used, for example, to measure the position of a stage of a mechanical apparatus. The application of incremental encoders to the fields of machine tools and robots is particularly limited, because incremental encoders cannot obtain absolute position information without first detecting an origin. On the other hand, absolute encoders do not need to detect an origin because they may obtain absolute position information.
Typically, to obtain a position of an object in two dimensions (in the X and Y directions) using an absolute encoder, two absolute linear encoders are attached to respective orthogonal side faces of a stage. However, it is often required to mount a two-dimensional scale on one surface, such as an upper or lower surface, of the stage. As a two-dimensional scale having absolute codes, Japanese Patent Laid-Open No. 11-248489 discloses one in which two-dimensional patterns used to determine absolute positions are discretely arranged in a plane. In the two-dimensional scale disclosed in Japanese Patent Laid-Open No. 11-248489, to assign absolute position information to each grid region, the grid region is divided into a plurality of sub-regions (elements). Then, one of two types of code information, either white or black, is assigned to each sub-region.
PCT Japanese Translation Patent Publication No. 2008-525783 also discloses a two-dimensional scale in which two-dimensional patterns used to determine absolute positions are discretely arranged in a plane. In the scale disclosed in PCT Japanese Translation Patent Publication No. 2008-525783, color patterns of red, blue, green, and the like are arranged around each of measurement points spaced at regular intervals, so that absolute position information is expressed by the arrangement of the color patterns. In a two-dimensional scale disclosed in Japanese Patent Laid-Open No. 2004-333498, a plane is divided into grid regions, each of which includes a two-dimensional quasi-random pattern portion and a two-dimensional code portion. An absolute encoder disclosed in Japanese Patent Laid-Open No. 2004-333498 simultaneously detects two-dimensional quasi-random pattern portions and two-dimensional code portions, and determines a two-dimensional absolute position from the resulting two-dimensional image pattern.
The two-dimensional absolute encoders disclosed in Japanese Patent Laid-Open No. 11-248489, PCT Japanese Translation Patent Publication No. 2008-525783, and Japanese Patent Laid-Open No. 2004-333498 all require a scale on which two-dimensional patterns each representing an absolute position are discretely arranged, and thus cannot easily achieve high resolution comparable to that of incremental encoders.